MIL-003 Console interface.
- Add calculator() with recursive restart: y continues with the result,
n starts a new calculation, q quits
- Show one ASCII title at start: a calculator with the lettering to
its right
- Re-prompt on invalid numbers, operations and answers; restart on
division by zero
- Add __main__.py for python -m calc and the calc console script
- Add interface tests with input() fed through monkeypatch
Closes#22Closes#23Closes#24Closes#25
Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4